Justification is a term taken from the law courts. It is the picture of the guilty criminal standing before the judge for sentencing, and then learning that because of what someone else has done for him, he is to be set free.
Propitiation belongs to the language of sacrifice. Paul contends that the sacrifice of Christ gives content to the sacrifices of the Old Covenant and not as we suppose that the old sacrifices cast light on the work of Christ. Sacrifice in its deepest sense is not that which man offers to God, but what God offers to man in Christ.